A magnetic memory or magnetic random access memory (MRAM) is a non-volatile memory in which a high speed operation and the infinite number of times of rewrite operations can be carried out. Already, its practical use is partially started. Also, a development for improving versatility is advanced. In the MRAM, a magnetic material is used as a storing element, and a datum is stored in correspondence to the orientation of the magnetization of the magnetic material. As a method of switching the orientation of the magnetization of the magnetic material, several methods are proposed, and they are common in the use of a current. How much the write current can be reduced is very important for practical use of a MRAM. According to Non-Patent Literature 1, the reduction to 0.5 mA or less is required, and further preferably, the reduction to 0.2 mA or less is required. This is because the minimum layout can be attained in a 2T-1MTJ (two transistors—one magnetic tunnel junction) circuit proposed in the Non-Patent Literature 1, when the write current can be reduced to about 0.2 mA, and a cost performance equivalent to existing volatile memories such as DRAM and SRAM can be attained.
The most typical method among methods of writing data into the MRAM, is a method of arranging interconnections for a write operation around a magnetic memory device and switching the direction of the magnetization of the magnetic memory device by a magnetic field generated by supplying current to the interconnections. In this method, the direction of the magnetization is inverted by the magnetic field. Thus, in principle, the write operation can be attained in 1 nano-second or less, which is preferable for attaining the high speed MRAM. However, the magnetic field for switching the direction of the magnetization of the magnetic material in which thermal stability and disturbance magnetic field durability are reserved becomes about several 10 Oe (oersted). Therefore, the current of several mA is required to generate such magnetic field. In this case, a chip area has to be large, and a consumed power amount required for a write operation also increases. Therefore, this random access memory is inferior to other random access memories in the competitiveness. In addition to this, when the element has a miniaturized shape, the write current is further increased, which is not preferable from the viewpoint of scaling.
In recent years, as means for solving such problems, the following two methods are proposed. The first method uses a spin injection magnetization inversion. In this method, a stacked film is formed by a first magnetic layer (magnetization free layer) having an invertible magnetization; and a second magnetic layer (reference layer) which is electrically connected to the first magnetic layer and whose magnetization is fixed; and the magnetization of the first magnetic layer (magnetization free layer) is inverted by using a mutual action between conductive electrons spin-polarized when current is passed between the second magnetic layer (reference layer) and the first magnetic layer (magnetization free layer), and local electrons in the first magnetic layer (magnetization free layer). At a time of read, a magnetic resistance effect is used between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer). Thus, the MRAM that uses the spin injection magnetization inversion method is an element of two terminals.
The spin injection magnetization inversion occurs at a time of a certain current density or more. Thus, when the size of the element is small, the current for write is reduced. That is, the spin injection magnetization inversion method is superior in scaling property. However, an insulating layer is typically provided between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer), and at the time of write, a relatively large current must be supplied to the insulating layer. Thus, this results in problems of rewrite durability and reliability. Also, since a current path for the write operation and a current path for the read operation are same, an erroneous write operation is made anxious at the time of read. In this way, although the spin injection magnetization inversion is superior in the scaling property, several bars exist against a practical use.
On the other hand, the second method, namely, a magnetization inversion method that uses a current driven magnetic domain wall motion phenomenon can solve the above problems existing in the spin injection magnetization inversion. The MRAM that uses the current driven magnetic domain wall motion phenomenon is disclosed in, for example, Patent Literature 1. In the MRAM using the current driven magnetic domain wall motion phenomenon, the magnetizations at both ends of the first magnetic layer (magnetization free layer) that typically has the invertible magnetization are fixed so as to be substantially anti-parallel to each other. At a time of the foregoing magnetization arrangement, a magnetic domain wall is introduced into the first magnetic layer. Here, as reported in a Non-Patent Literature 2, when a current is supplied to a direction passing through the magnetic domain wall, the magnetic domain wall is moved to the direction of a flow of the conductive electrons. Thus, it is possible to write by supplying the current into the first magnetic layer (magnetization free layer).
At a time of reading data, a magnetic tunnel junction (MTJ) provided in a region in which the magnetic domain wall is moved is used to carry out the read operation through the magnetic resistance effect. Thus, the MRAM using the current driven magnetic domain wall motion method serves as an element of three terminals and also matches with the 2T-1MTJ configuration proposed in the above Non-Patent Literature 1. Since the current driven magnetic domain wall motion occurs at a time of a certain current density or more, this has the scaling property similarly to the spin injection magnetization inversion. In addition to this, in the MRAM element that uses the current driven magnetic domain wall motion, the write current does not flow through the insulating layer inside the magnetic tunnel junction, and the path of the write current and the path of the read current are different. For this reason, the above problems in the spin injection magnetization inversion are solved.
Also, in the Non-Patent Literature 2, about 1×1018 [A/cm2] is required as a current density necessary for a current driven magnetic domain wall motion. In this case, for example, when a width of the layer (magnetization free layer) in which the magnetic domain wall motion occurs is assumed to be 100 nm and a film thickness is assumed to be 10 nm, a write current becomes 1 mA. This can not satisfy a condition of the above write current. On the other hand, as described in a Non-Patent Literature 3, the write current can be reduced to a sufficiently small value, by using a material having a perpendicular magnetic anisotropy for a ferromagnetic layer (magnetization free layer) in which the current driven magnetic domain wall motion occurs. In view of the above, when the current driven magnetic domain wall motion is used to manufacture a MRAM, it is preferable to use the ferromagnetic material having the perpendicular magnetic anisotropy as the layer (magnetization free layer) in which the magnetic domain wall motion occurs.